Where did the material node go in Blender 2.8?
In 2.8, nodes are the constituent parts of a material. Shaders, textures, color and all the other nodes make up a material. If you want to create a new material from the shader editor, click the "New" button at the bottom or choose from existing materials to edit.
 
Or you can click the material tab on the side and call up the material panel.